All enzymes containing either tungsten or molybdenum, with the sole exception of nitrogenase, bind the metal in the form of a complex with the unique dithiolene group of a novel pterin termed molybdopterin. Molybdopterin itself is the invariant moiety of a family of compounds in which the terminal phosphate group is present either as a monophosphate or in pyrophosphate linkage to GMP, CMP, AMP or IMP. In humans the genetic deficiency of either sulfite oxidase, a molybdoenzyme, or of molybdopterin, the pterin component of the Mo cofactor of sulfite oxidase, leads to severe neurological defects and in most cases death in infancy. A major goal of this project is to unravel in complete detail the pathway of biosynthesis of molybdopterin. It was earlier demonstrated that the immediate precursor of molybdopterin is a pterin cyclic phosphate in both E. coli and humans. The studies proposed here are aimed at identifying the primary precursor and all subsequent intermediates of the molybdopterin biosynthetic pathway. These studies in E. coli are of great significance per se since a crucial aspect of the pathway is the mechanism of mobilization of sulfur in sulfur-containing structures including several vitamins and Fe/S centers. Because the pterin structure is unique, all of the reactions leading to its biosynthesis are likely to be mechanistically novel and intriguing. The results of these studies should also provide the basis for delineating the molybdopterin biosynthetic pathway in humans, including the question whether the pterin is synthesized de novo in human tissues or whether it is formed from a dietarily derived precursor. Knowledge that molybdopterin is synthesized from precursor Z in humans could not have been derived without the characterization of the precursor in E. coli. Molybdoenzymes display a wide diversity in the reactions catalyzed. In order to understand the basis of this diversity, it is necessary to carry out structure-function studies on a number of the enzymes. The simplest of the molybdoenzymes is DMSO reductase, an enzyme containing the Mo cofactor as the sole prosthetic group. We will continue to carry out extensive structural studies on the enzyme and on sulfite oxidase, using X-ray crystallography, EXAFS, RR spectroscopy and MCD spectroscopy. The cloned genes of the enzymes will be used to generate site-specific mutants that can also be investigated using these techniques. These studies will subsequently be extended to other molybdoenzymes.